1. Field of the Invention
The present invention relates to a resin coat piece provided in a bearing at a fitting surface with a housing to prevent occurrence of creep.
2. Description of the Prior Art
In case where a steel bearing is incorporated in a housing which has a great thermal expansion coefficient such as formed of an aluminum alloy, when the ambient temperature rises, an interference between fitting portions of an outer ring of the bearing and of the housing decreases due to a difference in the thermal expansion coefficients of the both. This results in the occurrence of so-called creep wherein the outer ring of the bearing turns relative to the housing.
To prevent the occurrence of such creep, the conventional arrangement is made such that a resin coat piece formed of a resin material is attached to the bearing with its fitting surface to the housing being continuously extended along the circumference of the bearing. In this arrangement, the frictional resistance of the resin coat piece is effective to prevent the bearing from turning relative to the housing even when the interference between the fitting portions decreases.
Examples of a usable resin material for forming the resin coat piece include polyamide 66 (PA66), polyamide 11 (PA11), polybutylene terephthalate (PBT) and the like.
The aforementioned resin materials (PA66, PA11, PBT) used for the conventional resin coat piece have properties as shown in the following Table 1. Based on these properties, each of the resin materials is evaluated as shown in Table 2. Incidentally, the circle in the Table 2 denotes "excellent", whereas the triangle denotes "relatively inferior".
As appreciated from Table 2, however, the resin materials have respective problems although they fully serve the purpose of preventing the creep. More specifically, PA66 has a great dimensional change due to water absorption. PA11 is incapable of withstanding continuous use for a long term at a temperature above 150.degree. C., for example. Furthermore, PA11 is more expensive and less versatile in general as compared with PBT and PA66. PBT, in turn, is incapable of withstanding continuous use for a long term at a temperature above 150.degree. C., for example.
TABLE 1 ______________________________________ Resin types Properties PA66 PA11 PBT ______________________________________ Specific gravity 1.13 1.04 1.31 Water absorption %! 8.5 2.5 0.4 Tensile strength Mpa! 83 56 52 Flexural strength 118 66 93 Mpa! Fusion point .degree. C.! 260 187 225 Thermal deformation 230 154 154 temperature .degree. C.! At load of 0.46 Mpa 8.1 15 10 Thermal expansion coefficient 10.sup.-5 /.degree. C.! Cost ratio 1 3.2 1.3 (based on PA66 as 1) ______________________________________
TABLE 2 ______________________________________ Evaluation Resin types item PA66 PA11 PBT ______________________________________ Creep torque .largecircle. .largecircle. .largecircle. Heat .largecircle. .DELTA. .DELTA. resistance Dimensional .DELTA. .largecircle. .largecircle. stability Cost .largecircle. .DELTA. .largecircle. ______________________________________